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Marshall PriestMarshall Priest 

How do I fix this error?

This app can't be installed.
There are problems that prevent this package from being installed.
Apex Classes(classes/OpportunityAlertController.cls) Missing feature: Installing this package requires the following feature and its associated permissions: Apex Classes
 
Waqar Hussain SFWaqar Hussain SF
I think you are using Professional Edition of Salesforce. 

To install an app with Apex in professional edition, your app has to be a managed package that is certified.  Only partners that meet the necessary requirements can have their app certified.

Another option is to upgrade your Salesforce edition to Enterprise edition.

make sense?
Marshall PriestMarshall Priest
I am attempting a module in Trailhead and this in the Developer org, not production. I have enabled Apex classes and have the Author Apex permission selected. Not sure why I am getting this error message